The present invention relates to industrial turbo engines, and more particularly to a method for controlling high-speed permanent magnet motors that are coupled to industrial turbo engines.
Permanent magnet synchronous motors (PMSM) are widely used in motion controls, electric vehicles, and industrial turbo generators (ITG). PMSMs are typically coupled with engines to provide assistance to the engines at lower engine speeds. Because a turbo engine cannot start itself at zero speed, PMSMs using a starter inverter are needed to assist the turbo engine in accelerating to about half of its rated speed. At this speed, the turbo engine has sufficient torque to be driven to its rated speed without a PMSM.
To start a turbo engine, and specifically an industrial turbo engine, a starter inverter drives the PMSM, which is coupled to the turbo engine, to an initial desired speed to warm up the turbo engine. The starter inverter, which converts direct current (DC) into alternating current (AC), takes power from either batteries or a rectified grid. After the turbo engine is warmed, the drive is then increased to a second desired speed, typically about half of the rated speed of the turbo engine. The turbo engine will then take over the process without the aid of the AC motor and continue accelerating to nominal speed.
An xe2x80x9copen-loopxe2x80x9d volts per hertz control method is a commonly used AC motor control method. The volts per hertz (V/f) algorithm is used to run the motor at variable speeds. The V/f algorithm is very simple and inexpensive to implement in comparison to more sophisticated AC motor control algorithms. The xe2x80x9copen-loopxe2x80x9d control method varies the three-phase output frequency by adjusting the slope, or the ratio between frequency and voltage. Normally, a single straight-line V/f ratio is determined that drives the AC motor to the turbo uncoupling speed, where the turbo engine can produce sufficient torque to accelerate on its own to a desired speed without the aid of an AC motor. No sensing equipment is needed, the calculations are simpler and a less powerful, less expensive motor control unit (MCU) can be used.
One problem with the xe2x80x9copen-loopxe2x80x9d control method is that due to a possible wide range of DC bus voltages and the short accelerating time, typically about 12 seconds, for an industrial turbo engine to accelerate from 0-33,000 rpm, a single straight-line voltage per hertz ratio calculated by the xe2x80x9copen-loopxe2x80x9d method does not give a satisfactory dynamic response. In addition, the xe2x80x9copen loopxe2x80x9d control method often results in larger line current outputs at high speeds, or unacceptable steady state current at low speeds if a short accelerating time is required.
It is thus an object of the present invention to determine a control method for the high-speed PMSM that overcomes the problems associated with the open-loop control methods.
It is an object of the present invention to provide a control method for a very high-speed permanent magnet motors that provides a volts per hertz (V/f) profile from zero speed to very high-speeds. The V/f profile factors in the wide range of DC bus values and minimizes the line to line current to a permanent magnet generator (PMG) motor.
In one aspect of the preferred invention, a permanent magnet synchronous motor (PMSM) is coupled to an industrial turbo engine. A battery or other DC power supply sends a DC voltage to a starter inverter, which modulates the DC bus voltage to a three-phase AC voltage, and sends it to the PMSM. The PMSM controller has a V/f algorithm that determines a V/f profile for the given DC bus voltage to drive the PMSM to a speed at which the industrial turbo engine has sufficient torque to accelerate without the aid of the PMSM motor, typically about one-half of the rated speed of the industrial turbo engine. The algorithm creates a V/f profile along a pathway where steady state and transient current are minimized. The V/f profile created from the V/f algorithm is a function of a calculated voltage-offset value, an actual low-speed rated speed value, and an actual high-speed rated speed value. The algorithm takes into account the wide range of potential DC bus voltages from the battery, which may have different charge levels at different times.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.